Developing generic strategies that are capable of driving multielectron processes are essential to realize important photocatalytic conversions. Here, we present the idea of introducing favorable catalyst–reactant interaction in achieving efficient photocatalytic regeneration of nicotinamide (NADH) cofactor by gold nanoparticles (AuNPs). The electrostatic attraction emanating from the ligands on the surface of NP increases the channeling and local concentration of NAD+ reactants around AuNP photocatalysts, thereby enhancing the probability of the electron transfer process. Detailed kinetics- and intensity-dependent studies confirm the involvement of multiple electron transfer from the AuNP photocatalyst to the NAD+ reactant. The photocatalytic performances of AuNPs presented here are comparable to or greater than most of the catalytic systems reported based on plasmonic NP, with the added advantage of being structurally less complex. The use of electrostatics mimics the underlying force involved in various enzyme catalysis, which can serve as a generic approach for other important artificial multielectron photocatalytic reactions as well.

The creation of matter with varying degrees of complexities and desired functions is one of the ultimate targets of self-assembly. The ability to regulate the complex interactions between the individual components is essential in achieving this target. In this direction, the initial success of controlling the pathways and final thermodynamic states of a self-assembly process is promising. Despite the progress made in the field, there has been a growing interest in pushing the limits of self-assembly processes. The main inception of this interest is that the intended self-assembled state, with varying complexities, may not be “at equilibrium (or at global minimum)”, rendering free energy minimization unsuitable to form the desired product. Thus, we believe that a thorough understanding of the design principles as well as the ability to predict the outcome of a self-assembly process is essential to form a collection of the next generation of complex matter. The present review highlights the potent role of finely tuned interparticle interactions in nanomaterials to achieve the preferred self-assembled structures with the desired properties. We believe that bringing the design and prediction to nanoparticle self-assembly processes will have a similar effect as retrosynthesis had on the logic of chemical synthesis. Along with the guiding principles, the review gives a summary of the different types of products created from nanoparticle assemblies and the functional properties emerging from them. Finally, we highlight the reasonable expectations from the field and the challenges lying ahead in the creation of complex and evolvable matter.

Scheme 1. A schematic representation of the different classes of chemical transformations solely photocatalyzed by plasmonic NPs.

The underlying power of "interplay of forces" in controlling the properties and functions at the nanoscale is highlighted in this perspective. This interplay is achieved by installing attractive and repulsive forces, via proper ligand chemistry, which will guide the nanomaterials to interact with their surroundings as per the need. Along with improving the existing properties, the balancing of attractive and repulsive forces holds the prospects of imparting newer functions as well. The concept of "ligand-directed interplay of forces" is extensively practiced by our group and others for addressing many challenges in the areas of self-assembly, catalysis, light harvesting, and nanomedicine. The journey has been rewarding so far in terms of achieving many important feats in nanoscience, such as selfassembly under equilibrium and nonequilibrium regimes, outplaying ligand poisoning in nanocatalysis, channelizing the flow of energy and electron in donor−acceptor systems, multicolor photopatterning using a single nanohybrid system, biospecific targeting and therapy, and so on. As evident from this perspective, the diversity of the areas benefited showcases the breadth and depth of the impact that surface ligands and interplay of forces can have in material science. Furthermore, the implementation of the "ligand of choice" approach is one way to realize distinct and specific functions from a limited set of nanomaterials. All the examples of "liganddirected studies" mentioned in this perspective are based on the regulation of, primarily, electrostatic forces emanating from the charged surface ligands. Thus, it will be logical to try various combinations of ligands and forces, which means that the area of "ligand-directed nanochemistry" will keep on advancing beyond our predictions. Looking forward, there is plenty of room at the NP surface.

Instant hydrogen evolution occurs on shining solar light on to an artificial leaf device made of TiO2/Mn–CdS with a Pt or NiCu-alloy co-catalyst.